Multi-phase metering systems are widely used in the oil and gas and petrochemical industries. In the oil and gas industry it is necessary to measure the flow rates of oil, gas and water, all of which are produced from the sub-surface reservoir to surface via “production wells” drilled in the earth.
There are a number of current systems available to do this but few, if any, of them are either compact enough, or can be constructed “in-line”, to allow them to be used within the well bore, as well as at the surface.
It is known that two-phase hydrocarbon/water flow metering is achieved by the use of a volume fraction determining device in addition to a venturi. The fraction measurement device for hydrocarbon/water usually, but not always, takes advantage of differing electro-magnetic properties of the two phases to determine the volumetric phase fractions. Existing downhole water fraction meters allow the metering of the two-phase fractions (oil-water or gas-water) over the whole of the water fraction range from 0 to 1—for example, roxar MF1 water cut meter (Roxar, Norway), Fluenta MPFM 1900 (Fluenta, Norway). It is also known that it is possible to measure the gas fraction, otherwise known as the void fraction, by the use of radio-active density measurement devices.
By using the above two techniques along with flow velocity measurements derived from the cross-correlation of data from sensors spatially separated along the flow paths, it is possible to construct a system which will measure three-phase flow.
However, the use of radio-active sources has two principal disadvantages. Firstly, the sources require careful and substantial packaging to mitigate the health and safety implications of using such sources and, secondly, the use of such sources currently installed in oil and gas wells is still not an accepted practice throughout the oil and gas industry.
An object of the present invention is to provide an improved apparatus and method for measuring the flow and multi-phase fluids, particularly from production wells.